Of course it's a photo. All photography involves varying degrees of processing and subjectivity with respect to the information that is presented or discarded. Stacking may be somewhat less intuitive than a single frame, but conceptually it's no different from a long exposure - it's just selecting the moments of best seeing (minimal distortion from the atmosphere) to include in that long exposure. 400 frames of video at 60 fps is roughly analogous to a 6.7 second exposure in terms of signal vs noise.

Your typical single-frame exposure involves a great number of processing operations and filtering that may be unknown to most people, but still define the "look" of the end product (which is nowhere close to a 1:1 representation of the light entering the lens).